"We thought they were right and some of them came very early in the 20th century, so with the curators and the technological means at the time, all these indicated that they were fantastic and we thought that they were right," he told CNN.

It was only later, following advances in scholarship and the technology used to verify works of art, that they were shown to be fakes.

According to Salort-Pons, finding out that a work is a fake can be one of the worst moments in a curator's career.

"You think this work is a masterpiece, you use technology, you write about it and then you discover that it is a fake, so everything you have written is exposed and you seem like a fool, basically," he said.

But it's not just embarassing, it can be expensive too. Bolton Museum in the UK, for example, paid £440,000 ($694,183) in 2003 for a phoney ancient Egyptian statue by a notorious family of forgers known the Greenhalghs.

But many forgers are not just in it for the money, according to one expert.

Noah Charney is the founder of ARCA, the Association for Research into Crimes against Art, a non-profit think tank and research group on issues in art crime based in Amelia, Italy, which also runs a Masters program on the subject.

He told CNN: "Most of them that we know of were initially trying to be artists themselves, their original creative works were dismissed at some point in the early part of their career."

"So the primary motivation for most art forgers really is sort of passive-aggressive revenge, with financial motivation taking a very much secondary role," he continued.

While most forgers are artists, he said, some are art conservators too, so they are skilled at getting around the scientific techniques used to verify an artwork.

And not only do they produce counterfeit artworks; they can also produce convincing counterfeit documents verifying their bogus works.

With these skills, forgers and forgeries can sometimes go undetected for years, making it difficult to say whether or not the numbers of forgeries are rising.

Vernon Rapley is the former head of the Art and Antiquities unit for London's Metropolitan Police, credited with cracking the Greenhalgh case, and is now in charge of visitor services and security at the Victoria and Albert Museum in London.

"It's very hard to say that there are rising numbers," he said. "There's more investigation now and more awareness, because I think most forgers go undetected and have done for years."

The easiest thing for a forger to do, and the most intelligent, is to try to create a lost work of art.--Noah Charney, founding director of ARCA

However, he also said that the current economic climate has pushed more non-specialist people in the UK, for example, into investing in art and so creating a market for forgers.

The internet has been both a help and a hindrance to forgers. On the one hand, according to Rapley, they can use online platforms to sell forgeries.

On the other hand, according to Charney, the internet has helped circulate information about the location of works of art, making it a lot more difficult for forgers to claim they are in possession of one.

"The easiest thing for a forger to do, and the most intelligent, is to try to create a lost work of art," he said.

But while many forgers stick to either copying or creating works from the past, some are bold enough to try to forge works by living artists.

Rapley's last prosecution was of a man in the UK forging work by the British artist Tracey Emin, who sold 11 fakes on eBay for £26,000 ($41,000).

"It always amazed me that criminals were willing to fake or forge living artist's work," he said, "But actually a lot of these artists don't really know what they've produced."